Zen cart version 1.3.8a

Hi all,

I have products on the store that are for "Teacher Training". The products have TEXT input fields. I also have a question above the TEXT input fields that ask if they are taking the class or if they are purchasing a seat for someone. What php file do I have to edit in order to make sure that if they are purchasing for someone that they HAVE to fill out the text fields.

I am familiar with doing this using javascript on any normal website, but I don't know how to integrate the code in zen cart.